1. Technical Field
The present invention relates to switch circuits. More particularly, the present invention relates to capacitive touch switch circuits in audio devices.
2. Related Art
Capacitive touch switches are an alternative to mechanical switches for providing user control inputs to various devices, including high voltage home appliances and lighting fixtures. With a capacitive touch switch, a user merely touches a capacitive area on the device to initiate a switching action, rather than moving a mechanical switch. Capacitive touch switches usually improve the ease of use of a device by users, especially those users who may have limited rotational movement or limited motor skills.
A capacitive touch switch generally utilizes a capacitive electric circuit that is responsive to the dielectric properties of a human body. A capacitor may be formed by disposing two metal plates in parallel with each other and separated by a distance D. Each plate has an area A. Thus, the capacitance of the capacitor may be calculated by the equation:
                              C          =                      K            ⁢                                          8.85                ×                                  10                                      -                    14                                                  ⁢                A                            D                                      ,                            (                  Eq          .                                          ⁢          1                )            where K represents the dielectric constant of the material between the plates, A represents the plate area in square centimeters and D represents the distance between the plates in centimeters.
The capacitance of a capacitor increases when a material that does not conduct electricity is disposed between the parallel metal plates. The ratio of the capacitance before and after the material is placed between the two plates is the dielectric constant K of the material. The dielectric constant of a vacuum KV is about 1 while the dielectric constant of distilled water KW is about 80. Thus, objects containing water, such as the human body, would increase the capacitance of the capacitor.
Because of the human body's dielectric properties, a capacitive touch switch may be activated by the touch of a finger or other body part on or near a capacitive touch area. An electronic circuit may be configured to operate by monitoring the capacitance change in the capacitive touch area. Thus, placing a human finger on or near the capacitive touch area forms an electrical interface that produces a measurable capacitance change.
However, the measured change in capacitance may be adversely affected by the environmental factors of the application for the capacitive touch switch. Factors that may affect the measured capacitance change when a human finger is on or near the capacitive touch area include the type of floor the human is standing on and the type of shoes he or she is wearing. The measured capacitance change may increase with bare concrete floors and thin-soled shoes. The measured capacitance change may decrease with carpeting and with thick-soled shoes. Furthermore, nearby metal shelves, tables, light fixtures and electrical wires also may vary the measured capacitance change caused by the human body. There also may be increased electromagnetic interference when increased capacitance is introduced into a particular environment from other sources such as lighting, electronic devices and the like. In many environments, at home, the office, or elsewhere, fluorescent lighting is a source of light. The 120 Hz signals generated by fluorescent and similar lighting may cause electromagnetic interference that may affect the operation of capacitive touch switches and may degrade the sound quality from an audio device.
Some applications that operate at lower voltages may be more sensitive to these environmental variations, especially electromagnetic interference. In applications such as audio devices and other devices for reproducing signals of higher quality, these environmental factors may interfere, alter, and/or degrade the signal quality of the measured capacitance change. Audio devices are systems and components that reproduce and/or assist in the reproduction of sound from electrical signals. Audio devices include sound reproduction equipment such as amplifiers, tuners, receivers, speakers, and the like. Audio devices also include systems and components that incorporate sound reproduction equipment such as personal computers, audiovisual devices and the like.